An Interesting Poster to look at from the Tri Alpha Energy Team in California

Density profiles of C-2W High Performance FRC
M Beall, E Parke, J Kinley, R Smith, M Kaur
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Abstract
In TAE Technologies’ current experimental device, C-2W (also called “Norman”) [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams, advanced divertors, end bias electrodes, and an active plasma control system. The performance of the fast ion-dominated FRC and the effects on it from beam injection are examined using a 14-chord interferometer array in the 432.5 um far-infrared (FIR) range. Equilibrium density profiles from the array, together with other diagnostic measurements, are presented to compare early C-2W discharges and new high-performance regime plasmas.
[1] H. Gota et al., Nucl. Fusion 59, 112009 (2019).
FIR interferometer
 The primary density diagnostic for the experiment is the Far-Infrared (FIR) interferometer.
 The system has 14 chords at 433μm sampling the plasma every 8 cm, with a sensitivity of 1016 m-2 at >1 MHz bandwidth, allowing observation of a wide range of plasma modes
 A third laser cavity was recently installed allowing for simultaneous polarimetry and interferometry
 Half the chords are tilted 15o towards the magnetic axis to allow for measurement of Bz
Particle Confinement
 Previous discharges always exhibited primarily negative concavity density traces
 New regime discharges confine core plasma sufficiently to achieve steady-state conditions for much longer than previous records
Raw density traces show increased fluctuation activity as a result of enhanced energetics, but plasma remains highly robust to bulk modes
See E. Trask poster UP10.00125
Profile Inversion
 In order to determine radial electron density from the line-integrated density measured by FIR, Abel inversion is used
𝟏 ∞𝒅𝑭 𝒅𝒚 𝒏𝒓=−𝒓 𝟐𝟐
Profile Evolution Early regime
Diagnostic Comparison
 FIR is a primary baseline diagnostic that provides consistent information about plasma behavior with high spatial and temporal resolution
 Shown at top right, the density gradient and the calculated excluded flux radius from magnetics track each other quite well
 Shown bottom right, the FIR density exceeds that measured by Thomson due to lack of information about edge density, but otherwise matches the profile structure well
Fast Ion Population
 Plasma shots could be very hollow with most density near seperatrix
 Comparably high trapped flux
 However, most energy generated in
the formation process; density and flux decay over the course of the shot
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New regime
High-performance discharges generate extremely hollow profiles with very sharp peaks near seperatrix
Plasma size and excluded flux radius increase over the course of the shot
See E. Parke poster UP10.00133
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 Inversion can be performed numerically using “Onion-peeling” method, by subdividing the plasma into radial sections 𝝏r, and determining the spatial matrix generating them
Line-integrated density 𝑳 = 𝝏𝒍 ∗ 𝑹 where R is the density at 𝟏 𝟏𝟏 𝟏 1
the outermost edge, and 𝝏𝒍𝟏𝟏 is the path length component through it
𝑳𝟐 =𝝏𝒍𝟐𝟏 ∗𝑹𝟏 +𝝏𝒍𝟐𝟐 ∗𝑹𝟐,wherethelengthtermsarethe components of L2 through R1 and R2 respectively
 In general, the 𝝏𝒍 terms can generate a matrix 𝑴 such that 𝑳=𝑴∙𝑹
 This matrix is triangular, and therefore trivially invertible
𝝅 𝒅𝒚 𝒚 −𝒓
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Reproducibility
High-performance FRCs are extremely robust and consistent across many shots
Peak density, stability and plasma structure far less dependent on external parameters and dynamics of formation process
Plotted below are averages and standard deviations of approximately 50000 timeslices of radial density profiles from historically high performing discharges compared to new regime discharges
 Plasma structure is highly dependent on input of fast ions from neutral beam injection
 As shown in the figure to the right, triggering 2 of 8 beams late in a discharge results in a rapid and pronounced increase in hollowness and overall density
 Behavior is consistent with rapid and efficient beam capture producing a fast-ion dominated plasma, also reflected in the increase neutron flux, shown below
See R. Magee poster UP10.00136
61th Annual Meeting of the APS Division of Plasma Physics, Fort Lauderdale, Florida, October 21-25, 2019